One type of magnetic storage medium is known as a magnetic memory disc. In general, this article comprises a substrate disc of non-magnetic material, a thin film of a magnetic material, a surface layer of a hard substance to protect the magnetic film from damage and, often a thin surface film of a lubricant. The substrate material usually used in magnetic memory discs is any of several aluminum alloys. Aluminum is preferred for this purpose because of properties such as lightness, strength and resistance to moderately high temperatures. A suitable alloy is one containing 5.1- 6.1% zinc, 2.1- 2.8% magnesium, 1.2- 2% copper, 0.18- 0.4% chromium and the balance aluminum.
One type of magnetic film commonly used in magnetic memory discs is composed of a cobalt-phosphorus alloy because of its desirable magnetic properties. The magnetic material should be one that can be deposited as a thin film since thin films are conducive to obtaining high packing density of stored information.
However, it is not feasible to deposit the cobalt alloy directly upon the aluminum alloy substrate because the cobalt alloy must be deposited on a smooth, highly polished surface. The aluminum surface is too rough and cannot be made to take a sufficiently high surface polish for this purpose.
To provide a highly polished surface of non-magnetic material suitable for deposition of the cobalt alloy film, a relatively thick (about one mil) layer of non-magnetic nickel-phosphorus alloy is deposited on the aluminum alloy substrate and this layer is then given a high polish.
The magnetic cobalt alloy film is then deposited on the nickel-phosphorus alloy layer. It is important that the cobalt alloy layer be protected from all surface damage since any surface damage can result in loss of parts of the recorded information.
Layers of various protective materials such as plastics, or metals such as rhodium, have been tried to protect the cobalt alloy layer from surface damage. Rhodium has desirable physical characteristics for this purpose but it is deposited from an acid solution that attacks the cobalt alloy layer. In seeking a better protective material that is not detrimental to the cobalt alloy layer, a layer of cobalt oxides has been proposed. Cobalt oxides are hard enough, adherent enough and otherwise have suitable properties for the intended purpose. However, the layer of cobalt oxides is formed by heating the deposited cobalt alloy layer at a temperature of about 210.degree. - 290.degree.C under controlled humidity conditions. At these temperatures, the underlying non-magnetic nickel-phosphorus layer becomes magnetic and this undesirably affects the magnetic properties of the cobalt alloy layers as a storage medium.
It would be desirable to have an otherwise suitable non-magnetic layer beneath the cobalt alloy layer that does not become magnetic at the temperatures used to form a cobalt oxide layer from cobalt. It would also be desirable to have such a layer that can be deposited by an inexpensive electroless deposition process rather than one requiring a more expensive process such as evaporation or sputtering.
One of the many other uses for electrolessly deposited nickel-phosphorus layers is for resistors in "thin" film hybrid circuits or for resistors in read-only memory matrix applications. Resistors having a rather wide range of resistances can be prepared by this method and the resistors so produced can be made to have small temperature coefficients of resistance depending upon conditions of deposition. However, in the past, it has been found that a heat treatment of these nickel-phosphorus films at temperatures of about 150.degree. to 250.degree. C for from 1 to 15 hours is necessary to stabilize both the resistances and the temperature coefficients of these resistors. It would be desirable to be able to eliminate the need for the heat treatment.
The present invention is based on the discovery that certain alloys of nickel, phosphorus and boron have properties which make these materials especially suitable for the non-magnetic layer beneath the magnetic cobalt layer in a memory disc and also cause these alloys to be particularly attractive for film type resistors in hybrid electronic circuits. Non-magnetic alloys containing about 0.1 to about 0.7 wt. percent boron, about 6-16 wt. percent phosphorus and the balance nickel have been found to remain substantially non-magnetic when heat treated. Alloys containing 0.1 to 0.7 wt. percent boron, 5 to 16 wt. percent phosphorus and the balance nickel have been found to have lower temperature coefficients of resistivity than either alloys containing only nickel and phosphorus or alloys containing only nickel and boron.